This invention relates to prosthetic feet, and to prosthetic limbs employing those feet.
As is well known in the art, an individual""s or an amputee""s gait during ambulation or other movement has several discrete phases, of which three are particularly significant. The first phase, known as heel strike, occurs when the heel contacts the surface along which the individual is moving. The second phase, known as flat foot, occurs when the foot is roughly parallel to the surface; and the third phase, known as toe-off, occurs when the heel of the foot is lifted from the surface and only the toe portion of the foot bears upon the surface to propel the individual forward.
A variety of different types of prosthetic feet have previously been proposed. One of the earlier examples of a previously proposed foot is shown in FIG. 1 and described in detail in United Kingdom Patent No. 2187102. As shown in FIG. 1, the foot 10 comprises a keel 12 that is fixedly attached to a plate 14 joined to the cosmesis 16 of the foot. A spring member 18, 20 is sandwiched between the keel 12 and the mounting plate 14. The keel 12 is provided with cantilevered snubber arms 22, 24 that serve to limit deflection of the spring member during ambulation. The spring arms serve to absorb energy on heel strike, and to release that energy (or at least a proportion thereof) on toe-off to facilitate ambulation of the amputee.
Whilst the arrangement shown in FIG. 1 provides many advantages over older systems, it has subsequently been noted that the device can cause discomfort to some amputees in certain circumstances. For example, it has been noted that the co-operating spring member 18 and snubber arm 22 can cause jarring on heel strike. It has also been noted that a significant amount of the energy stored during heel strike is lost, and thus that less stored energy is available to aid the amputee during toe-off.
United Kingdom Patent Application No. 2311466 discloses one system that purports to alleviate these and other problems. The device disclosed in the publication employs a compressible energy storage device which can take the form of an elastomeric or rubber buffer or a gas-filled strut, for example. The device employs a heel spring and a toe spring that are each pivotally mounted to a frame within which the energy storage means is provided. A complicated locking arrangement is provided to lock the energy storage means in a compressed state when it has been compressed during heel strike, and to release the energy storage means from that locked compressed state during toe-off. This arrangement, whilst going some way to providing better energy recovery during ambulation, suffers from the disadvantages that it is unduly complicated, potentially noisy and, by virtue of its complicated construction, possibly prone to mechanical failure.
Another system that purports to aid energy recovery is disclosed in International Patent Publication No. WO98/53769. This publication discloses a device comprising an ankle member to which a heel member is pivotally attached, and a toe member pivotally attached to the heel member. The heel member includes an arm that extends beyond the pivot point between the ankle member and the toe member. Rubber buffers are provided between the heel member and the ankle member, between the heel member arm and the ankle member, and between the heel member arm and the toe member. Ambulation of the amputee causes compression of the various buffers and toe-off is assisted as these buffers decompress. The publication discloses a variety of different buffers that can serve to reduce the operating noise of the prosthesis.
Whilst this arrangement is advantageous over that disclosed in United Kingdom Patent Application No. 2311466, it still demonstrates a significant energy loss during ambulation. Furthermore, it has also been noted that problems can arise with rapid degradation of the buffers particularly when employed in prostheses fitted to amputees of over 90 kg in weight. These problems are exacerbated by the fact that the prosthesis can become significantly warmer during use, the increase in temperature causing a corresponding increase in the rate of buffer decay.
Aspects of the present invention variously seek to further improve energy recovery during ambulation, and to avoid problems associated with the use of the prosthesis by heavier amputees (i.e. by amputees of over 90 kg in weight). It will be understood, of course, that the prosthesis of the present invention can also provide significant advantages to amputees of less that 90 kg in weight.
In accordance with an aspect of the invention, there is provided a prosthetic foot comprising: a spring assembly having an anterior component and a posterior component; a snubber pivotally connected to said spring assembly for limiting movement of said spring assembly during ambulation; and anterior and posterior shock absorbers provided between respective components of said spring assembly and said snubber; wherein the arrangement of said posterior shock absorber, said spring assembly and said snubber is such that ambulation results in the generation of a compressive force in said posterior shock absorber during a heel strike phase and a tensile force in said posterior shock absorber during a toe-off phase. It should be noted that provision of an anterior shock absorber is optional.
This arrangement is advantageous as it allows energy recovery from the posterior shock absorber to occur both in the heel strike and toe-off phases thereby increasing the quantity of energy that can be recovered during toe-off.
Preferably, the anterior shock absorber is replaceable, or removable from said prosthesis for replacement. Preferably, the posterior shock absorber is replaceable, or removable from said prosthesis fore replacement. Preferably, the posterior shock absorber is releasably attached to said spring assembly and said snubber.
Preferably, the spring assembly comprises a pair of resiliently flexible members each of which have a pair of cantilevered arms. One of the pair of flexible members may be shorter than the other. The flexible members are preferably releasably attached to a keel that forms the remainder of said spring assembly.
Alternatively, the spring assembly may comprise a secondary resiliently flexible member fixedly attached to a keel, and one or more resiliently flexible sole members attached to said secondary flexible member, said keel being pivotally attached to said snubber.
A further aspect of the invention provides a prosthetic foot wherein at least one of said anterior and posterior shock absorbers are of a multi-layer construction.
Preferably, at least one of said anterior and posterior shock absorbers are of a multi-layer construction comprising a sandwich of compressible and substantially non-compressible or rigid layers.
Preferably, said posterior shock absorber is less compressible than said anterior shock absorber.
A further aspect of the invention provides a shock absorber for a prosthetic foot, said shock absorber having a multi-layer construction comprising a sandwich of compressible or resiliently deformable layers and substantially non-compressible or rigid layers.
Preferably the compressible or resiliently deformable layers have a hardness in the region of 65 IHRD or 75 IHRD.
A further aspect of the invention provides, in combination, a prosthetic foot as described herein, a shock absorber as described herein as said anterior shock absorber, and a shock absorber as described herein as said posterior shock absorber. A further aspect of the invention provides a prosthetic leg comprising an exoskeleton and a prosthetic foot as described herein. A further aspect of the invention provides a prosthetic leg comprising an endoskeleton and a prosthetic foot as described herein. A further aspect of the invention provides a cosmesis for a prosthetic foot as described herein, said cosmesis comprising strengthening means in the vicinity of a toe region thereof. Preferably, the strengthening means comprises a member embedded in the sole of the cosmesis. The strengthening means may comprise a strip of material of greater stiffness than the remainder of the cosmesis.